A high power factor preregulator is frequently added at the power input of power electronic equipment to improve the harmonic content of the current waveform drawn from the line. In its most common implementation, as shown in FIG. 1, the high power factor preregulator takes the form of a "boost converter" which forces the current in an inductor 10 in series with the input line 11 to follow a reference voltage 12 derived from the line voltage itself. The control circuit 14 changes the duty cycle of the switch 16, normally a power transistor, to make the current signal 18 conform to the voltage reference 12. The current signal 18 may be monitored through a suitable current transducer 20.
Thus, if the high power factor preregulator stage is working properly, the input current waveform on line 11 is a replica of the input voltage waveform on line 11. If the input voltage is a perfect sinusoid, the voltage and current waveforms on the input side of the rectifier bridge 22 will be in-phase sine waves. If the input line contains voltage harmonics, they will appear as distortion components in the current wave form.
This high power factor preregulator stage is also commonly used to preregulate the output voltage, as shown in FIG. 2. The block diagram of FIG. 2 differs from that of FIG. 1 by the fact that the reference signal 12' for the current is not taken directly from the rectified line through a voltage divider at the output of the rectifier stage (as in FIG. 1), but instead is fed through a multiplier 24 that scales the reference signal according to the deviation of the output voltage from its desired value. This is accomplished through sampling the output voltage via a voltage divider stage 26 and feeding the output reference signal 28 to an error amplifier 30, the output of which is sampled by a sample/hold stage 32 to scale the multiplication operation.
In both implementations, if the input line 11 voltage increases, so does the reference voltage 12 or 12' and consequently, the input current. It follows that the power delivered to the output load increases with the square of the input voltage. This is an undesired consequence of this control method, since the purpose of the high power factor preregulator is either to make the load appear to the line as a resistor (current proportional to voltage and in phase with it) or to supply power to a load that is relatively constant and independent of line fluctuations while improving its power factor. In this latter case, if the input voltage goes up, the input current should go down.
Ultimately, this increase in output voltage is corrected by the voltage regulation loop shown in FIG. 2 and fed by reference signal 28. However, the bandwidth of this loop is kept purposely low, below 20 Hz, to prevent the ripple present at the output (twice line frequency) from interfering with the operation of the basic current waveform control of FIG. 1 and introducing harmonic distortion.
It will be appreciated that even with a very low bandwidth, current waveshape control and output voltage regulation pose conflicting requirements on the control circuit. This conflict is normally resolved with compromises that introduce some current waveform distortion and degrade output voltage regulation somewhat.
The sample and hold technique is frequently used to reduce the distortion introduced by the voltage regulation loop. Sampling is done at the instant in which the output voltage equals its average value and the output of the sample and hold circuit changes the input to the multiplier only at the beginning of each half cycle.
Several techniques are presently in use to achieve better overall performance or eliminate this conflict altogether. The addition of a power conditioning stage at the output of the high power factor preregulator shown in FIG. 1 would, of course, provide an additional degree of control freedom, thus allowing independent control of current waveshape and output voltage. This increases cost and reduces efficiency. Short of a full power conditioning stage, more complex circuits can improve overall performance by compromising cost and efficiency.
With only one power conditioning stage, as shown in FIG. 2, to eliminate the dependence of the output power from the input voltage, a feed forward loop can be introduced, as shown in FIG. 3, that divides the error in the voltage regulation loop by the square of the input voltage. The divider stage is shown at 32 in FIG. 3 and an input voltage reference signal 35 is provided from an additional voltage divider 34 to a squaring circuit 36. The output of the squaring circuit 36 is divided into the output from the error amplifier 30 to provide the divider output which scales the multiplier 24 for the reference voltage 12 fed to the control circuit 14. This method has the limitations normally associated with a multiplicity of arithmetic blocks (multiplier, divider and squarer), for example, scaling errors, offsets and drifts. This feed forward loop can be accomplished using a type UC3854 chip available from Unitrode Corp.
This same result can be obtained by making the gain of the multiplier stage of FIG. 2 a function of the input voltage according to the law 1/V.sup.2. This can be done with, for example, a (component) ML4821 chip from Microlinear.
An alternative method comprises developing a signal that is proportional to the peak value of the line voltage and after suitable processing, using it to modify the voltage reference derived from the line. For example, UCS3810 from Cherry Semiconductor may be used for this purpose.
As long as a single switch is used to perform the double function of current waveshaping and output voltage regulation, the conflict between the two goals mentioned above remains, however. As a result, the bandwidth of the voltage regulation loop must remain low and line or load transients will cause large swings in output voltage. These swings can only be corrected over a few cycles of the line frequency and can be damaging to the load. Hence, some additional control circuitry is frequently added to override the normal operation of the voltage regulation loop and force immediate corrective action at the expense of input current distortion.